In electrical audio amplification systems, resonant acoustical feedback results from the transmission and/or reflection of sound waves between a speaker and a microphone and the in-phase amplification of the electrical sound signals between the microphone and the speaker. Acoustic resonant properties vary greatly at different frequencies with different transmission, reflection and absorption properties of different rooms and with different positioning of microphones, speakers and other objects in rooms. When amplification or volume is set to a desired level, there often occurs acoustic resonant feedback at one or more frequencies. Acoustical resonant feedback, if not filtered to eliminate the resonant feedback, overwhelms the desired audio signal to produce an extremely loud, unpleasant tone.
A notch filter, or a band reject filter, is a well known device for attenuating electrical signals between any two specified frequencies while not appreciably affecting signals at other frequencies outside this band or channel. A notch filter tuned to a center frequency equal to a feedback frequency may be utilized for suppression of the feedback by holding the amplitude of the feedback signal below unity gain. However because the frequency of acoustical feedback is unpredictable and may occur at almost any frequency within the audio frequency spectrum extending from approximately 20 to 20,000 Hz, the frequency of the notch filter or filters in sound amplification systems must be individually selected for the particular rooms or locations of the microphones and speakers of the sound amplification systems. Also the required attenuation varies with different locations.
Graphic or parametric equalizers are often used in the electrical amplification circuit to suppress acoustical feedback. These equalizers employ a plurality of adjustable attenuators with respective bandpass filters, or adjustable notch filters, tuned to successive frequency bands or channels spanning the audio frequency range. By increasing the attenuation of the frequency band or bands containing the undesirable resonant feedback frequency or frequencies to reduce amplification the acoustical feedback can be eliminated.
In practical applications the operator of a graphic equalizer tries to equalize the sound system before the performance. After the speakers, microphones and amplifiers have been installed, the operator turns the volume of the amplifier up until feedback occurs. The operator then adjusts the controls that control the attenuation of the notch filters until the feedback is eliminated. Often several tries are required to get the right setting. It is not uncommon for more than one filter to be required for a single resonance if the resonance occurs between two adjacent bands. Next, the operator increases the volume of the amplifier until the next resonance occurs and repeats the process. This process is usually repeated until three or four resonant frequencies are attenuated.
Once the program begins, the operator must be vigilant in case new resonant frequencies occur during the program. This is common because microphones frequently are moved during a performance and a room full of people often has different acoustic characteristics than when it is empty.
In many cases, churches, schools, clubs, and small bands that use sound amplification equipment do not have trained sound system operators. The amplification system is often installed by a professional who adjusts the graphic equalizer for an empty room. Oftentimes, the unattended system resonates during a program, and an untrained user changes the equalizer until the resonance disappears. Changing the equalizer can result in excessive distortion of the music or the voice of the speaker using the microphone. The next day, a professional is called who equalizes again for an empty room. Thus, there is a continuing problem.
Graphic equalizers have limitations in the number and the bandwidth of the channels which they control. In expensive professional systems, the equalizer can have sixty-two channels wherein each channel covers one-sixth of an octave. Substantial attenuation of three or four channels can introduce substantial distortion of the sound spectrum. Such distortion is even more likely with less expensive systems employing fewer channels of greater bandwidth.
Adaptive suppression of acoustic resonant feedback is taught in the prior art as exemplified in U.S. Pat. No. 4,079,199 to Patronic, Jr., U.S. Pat. No. 4,091,236 to Chen, U.S. Pat. No. 4,165,445 to Brosow, U.S. Pat. No. 4,382,398 to O'Neill, U.S. Pat. No. 4,493,101 to Muraoka et al., U.S. Pat. No. 4,602,337 to Cox, U.S. Pat. No. 4,658,426 to Chabries et al., and U.S. Pat. No. 4,817,160 to De Koning et al. The adaptive systems include facilities for detecting the presence of resonant feedback and its frequency or the channel in which its frequency is found. Filtering is then performed in response to the resonant frequency detection. Several systems divide the electrical signal from the microphone into several channels spanning the audio spectrum and then lower the amplification or increase the attenuation of the channel or channels containing the resonant frequency or frequencies. Some systems utilize one or more frequency adjustable notch filters, such as switched capacitance filters, which are tuned to the resonant frequency or frequencies in response to the resonant frequency detection.
While the prior art adaptive systems provide automated alternatives to manually operated graphic equalizers, there still exists a need for automated acoustic feedback suppression with minimum sound distortion at a reasonable cost. The prior art adaptive systems generally have one or more deficiencies such as tending to produce excessive sound distortion, being excessively expensive, being excessively large, etc.